* Bacteria: These are single-celled organisms that lack a nucleus and other membrane-bound organelles. They have a wide range of metabolic capabilities and can be found in almost every environment on Earth.

* Archaea: These are single-celled organisms that were originally thought to be bacteria but are now known to be distinct. They are often found in extreme environments, such as hot springs, salt lakes, and deep-sea vents.

While both bacteria and archaea are prokaryotes, they have several key differences:

* Cell wall composition: Bacteria have peptidoglycan in their cell walls, while archaea do not.

* Membrane lipid structure: Bacteria have fatty acids in their cell membranes, while archaea have isoprenoids.

* Gene expression: The machinery used for gene expression (transcription and translation) in archaea is more similar to eukaryotes than to bacteria.

* Metabolic diversity: Archaea have unique metabolic pathways, including methanogenesis and the ability to use sulfur as an energy source.

These distinctions highlight the vast evolutionary diversity within the prokaryotic world.